Recently, extremely sensitive optical sensors have been constructed by exploiting an effect known as surface plasmon resonance (SPR). These sensors are capable of detecting the presence of a wide variety of materials in concentrations as low as picomoles per liter. SPR sensors have been constructed to detect many biomolecules including dinitrophenyl, keyhole limpet hemocyanin, .alpha.-Feto protein, IgE, IgG, bovine and human serum albumin, glucose, urea, avidin, lectin, DNA, RNA, hapten, HIV antibodies, human transferrin, and chymotrypsinogen. Additionally, SPR sensors have been built which detect chemicals such as polyazulene and various gases including halothane, tricloroethane and carbon tetrachloride.
An SPR sensor is constructed by sensitizing a surface of a substrate to a specific substance. Typically, the surface of the substrate is coated with a thin film of metal such as silver, gold or aluminum. Next a sensitizing layer, such as a monomolecular layer of complementary antigens, is covalently bonded to the surface of the thin film. In this manner, the thin film is capable of interacting with a predetermined chemical, biochemical or biological substance. When an SPR sensor is exposed to a sample that includes the targeted substance, the substance attaches to the sensitizing layer and changes the effective index of refraction at the surface of the sensor. Detection of the targeted substance is accomplished by observing the optical properties of the surface of the SPR sensor.
The most common SPR sensor involves exposing the surface of the sensor to a light beam through a glass prism. At a specific angle of incidence, known as the resonance angle, a component of the light beam's wavevector in the plane of the sensor surface matches a wavevector of a surface plasmon in the thin film, resulting in very efficient energy transfer and excitation of the surface plasmon in the thin film. As a result, at the resonance angle the reflected light from the surface of the sensor exhibits a sharp dip that is readily detected. When the targeted substance attaches to the surface of the sensor, a shift in the resonance angle occurs due to the change in the refractive index at the surface of the sensor. A quantitative measure of the concentration of the targeted substance can be calculated according to the magnitude of shift in the resonance angle.
SPR sensors have also been constructed using metallized diffraction gratings instead of prisms. For SPR grating sensors, resonance occurs when a component of the incident light polarization is perpendicular to the groove direction of the grating and the angle of incidence is appropriate for energy transfer and excitation of the thin metal film. As with prism-based sensors, a change in the amount of light reflected is observed when the angle of incidence equals the resonance angle. Previous SPR grating sensors have incorporated square-wave or sinusoidal groove profiles.
SPR grating sensors offer many benefits over SPR sensors having glass prisms including a thicker, more robust metal film. Furthermore, the grating period for an SPR grating sensor can be adjusted for any desired resonance angle. Despite these benefits, both current SPR grating sensors and prism-based sensors are susceptible to degradation due to oxidation of the metal film and its continuous exposure to the sample. For this reason, the thin metal film is usually constructed with a metal that tarnishes slowly, such as gold, rather than a highly sensitive metal which tarnishes more quickly, such as silver. Another disadvantage of current SPR sensors is that the metal film causes many biological substances to denature, thus leading to erroneous readings. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon understanding the present invention, there is a need in the art for an optical sensor having improved sensitivity and less susceptibility to degradation.